1. Technical Field of the Invention
The present invention relates to screens in particular plasma screens, and more precisely to the control of the cells of such a screen. However, the use of the invention is not limited to this type of screen. It can be applied for example to screens of liquid crystal type termed “LCDs”.
2. Description of Related Art
A plasma screen is a screen of matrix type, formed of cells disposed at the intersections of rows and columns. A cell comprises a cavity filled with a rare gas, and at least two control electrodes. To create a luminous dot on the screen by using a given cell, the cell is selected by applying a potential difference between these control electrodes, then ionization of the gas of the cell is triggered, generally by means of a third control electrode. This ionization is accompanied by an emission of ultraviolet rays. The luminous dot is created through excitation of a red, green or blue luminescent material by the emitted rays.
Conventionally, the control of a plasma screen essentially comprises two phases, namely an addressing phase in which the cells “pixels” which will have to be turned on and those which will have to be turned off are determined, as well as a display phase proper in which the cells that were selected in the addressing phase are read.
The addressing phase comprises sequential selection of the rows of the matrix. By way of example, the unselected rows are placed at a quiescent potential, for example 150 volts, while a selected row is brought to an activation potential, for example 0 volts. To select the chosen pixels of the selected row, the pixels that will have to be turned on by the display phase, the corresponding columns of the matrix are for example brought to a relatively high potential, for example 70 volts, by way of a power stage comprising MOS power transistors. The columns corresponding to the other pixels of the selected row, which will not have to be turned on, are brought to the 0 volts potential. Thus, the cells, which will have to be turned on, of the activated row see a columns-rows potential equal to about 70 volts, while the other cells of this row see a columns-rows potential equal to 0 volts. That said, it is also envisaged, in the addressing phase, by applying different potentials to the rows of the matrix, to apply a high potential to a column so as to select a pixel which will have to be turned off, and to apply a low potential to a column so as to select a pixel which will have to be turned on.
The selection and deselection of the column of the screen can be performed with the aid of a selection and deselection signal generated according to an energy recovery mode, also called the “charge sharing” mode. These modes comprise, for example, the mode commonly dubbed by the person skilled in the art “CSE” (“Charge Sharing Effect”) based on the use of a charging capacitor or else the “equalization mode” based on the principle of the flow of the potentials, which is for its part particularly suited to screens of LCD type. For example, the equalization mode consists in connecting the whole set of columns of the screen via their mid-point. Thus, when a column is deselected (therefore prior to the high state with a high value of potential), its potential will automatically flow towards a column to be selected, having a lower value of potential (since it is prior to the low state). Consequently a part of the energy necessary for the selection of a column originates from the columns which are simultaneously deselected.
As a variant, in the course of a selection, during the emission of a column's selection signal, according to the CSE mode, a necessary part of the charge originates from a capacitor incorporated into the control circuit of the screen. This mode is preferably used for plasma screens, but can also be employed for screens of LCD type as described in the document U.S. Pat. No. 5,852,426 (the disclosure of which is hereby incorporated by reference).
Stated otherwise, when a column is selected, part of the charge that is necessary for the formulation of the selection signal is transferred from the aforesaid capacitor to the column to be selected. Once such charge has been transferred, the selection signal has attained a first intermediate porch value, the occurrence of this intermediate porch being characteristic of the selection or deselection signals according to the charge sharing modes. The column control circuit output, coupled to the relevant control, is then switched over to the supply terminal of the circuit, so as to top the amplitude of the selection signal up to its maximum value, in general the value of the supply voltage.
Conversely, when the column is deselected, the output of the column control circuit is then coupled to the charging capacitor so as to charge it by restoring part of the charge to it. The deselection signal therefore goes from its maximum value to the intermediate porch value, then the column control circuit output coupled to the relevant column is switched to the earth of the circuit, in such a way that the deselection signal attains its minimum value.
The CSE mode makes it possible to reduce the energy consumption of the circuit, since part of the charge originates from a capacitor. The energy gain can reach as much as 50% with respect to a circuit operating without this mode.
However, it has been observed that depending on the data displayed on the columns of the screen, the temporal evolution characteristics of the selection or deselection signals vary. These variations can disturb the display of the data on the columns of the screen, and consequently the image displayed. For example, they can bring about a restriction in the time for which the column is kept selected or deselected, it then being possible for this time to be less than the minimum time necessary, to ensure selection or deselection of the column. This problem is particularly frequent when the columns are selected or deselected one after the other according to the mode commonly called “jitter” by the person skilled in the art.
These variations in the temporal evolution characteristics of the selection or deselection signals can also elongate their rise or fall time, to such a point that when switching the relevant column to the supply terminal or to the earth, the selection or deselection signal has not attained the value of the intermediate voltage allowing the realization of the intermediate porch, and therefore the complete discharging or charging of the charging capacitor.
A need exists for a method of controlling a matrix screen, using in particular a charge sharing mode for controlling the columns of this screen, in such a way that the temporal evolution characteristics of the column selection or deselection signals make it possible to select or to deselect the relevant column without disturbing the display of the data.
A further need exists for a method of controlling the columns so as to very precisely adjust the temporal evolution characteristics of their selection or deselection signals.
Yet another need exists for a device making it possible to implement this method, as well as a corresponding screen, in particular a plasma screen.